Comparison of abdominal muscle activity during a
single-legged hold in the hook-lying position on the floor and on a
round foam roll.

Abstract:

Context: To improve trunk stability or trunk muscle strength, many
athletic trainers and physiotherapists use various types of unstable
equipment for training. The round foam roll is one of those unstable
pieces of equipment and may be useful for improving trunk stability.

Objective: To assess the effect of the supporting surface (floor
versus round foam roll) on the activity of abdominal muscles during a
single-legged hold exercise performed in the hook-lying position on the
floor and on a round foam roll.

Design: Crossover study.

Setting: University research laboratory.

Patients or Other Participants: Nineteen healthy volunteers (11
men, 8 women) from a university population.

Intervention(s): The participants were instructed to perform a
single-legged hold exercise while in the hook-lying position on the
floor (stable surface) and on a round foam roll (unstable surface).

Main Outcome Measure(s): Surface electromyography (EMG) signals
were recorded from the bilateral rectus abdominis, internal oblique, and
external oblique muscles. Dependent variables were examined with a
paired t test.

Results: The EMG activities in all abdominal muscles were greater
during the single-legged hold exercise performed on the round foam roll
than on the stable surface.

Conclusions: The single-legged hold exercise in the hook-lying
position on an unstable supporting surface induced greater abdominal
muscle EMG amplitude than the same exercise performed on a stable
supporting surface. These results suggest that performing the
single-legged hold exercise while in the hook-lying position on a round
foam roll is useful for activating the abdominal muscles.

Trunk stability is essential to prevent lumbar compensa tory motion
(1, 2) and to reduce the intensity (3-5) and recurrence rate (6) of low
back pain. Trunk stability is maintained by passive, active, and neural
control subsystems. (7) The trunk muscles are coactivated through
integrated active and neural control subsystems to stabilize the trunk
and spinal segment. (7-9) Unlike the cervical spine, the lumbar spine
lacks flexor muscles just anterior to the vertebral body; thus, to
achieve trunk stability, it is essential to improve abdominal muscle
activity and coordination. (8, 9) Previous authors have suggested that
trunk stability can be improved with pelvic tilt, (10) abdominal
hollowing, (10) abdominal bracing, (10) curl-up, (11) bridging, (12) and
"dead-bug" exercises. (11) Unstable surfaces, such as a gym
bali or wobble board, have been used to increase the difficulty level of
trunk stability exercises. (13) Previous researchers (13-15) compared
the activity of the tnmk and abdominal muscles on unstable and stable
surfaces and demonstrated that abdominal muscle activity was greater on
the unstable surface. Rectus abdominis (RA) and external oblique (EO)
activity was greater when curl-up exercises were performed on unstable
surfaces compared with stable surfaces, (13) and Marshall and Murphy
(14) reported that activity of the RA muscle was greater during exercise
on the Swiss bali than on a stable surface. Similarly, Behm et al (15)
found that activity of the upper lumbar erector spinae, lumbosacral
erector spinae, transversus abdominis (TrA), and internal oblique (IO)
muscles during the chest press exercise was greater on an unstable
surface than on a stable surface.

A unilateral, active straight-leg raise in supine position can be
used to test lumbar spine stability in the supine position (1); lumbar
axial rotation may occur. (1) Furthermore, an asymmetric load on the
trunk induced by a unilateral single-legged hold exercise on an unstable
round foam roll causes more lumbar axial rotation, (16) but abdominal
muscle activity in this circumstance has not been reported. Therefore,
the aim of our study was to compare the abdominal muscle activity
measured during a unilateral single-legged hold exercise in a hook-lying
position on the floor and on a round foam roll. We hypothesized that
performing the exercise on a round foam roll would induce greater muscle
activity than would the same maneuver on the floor.

METHODS

Participants

Nineteen volunteers participated in this study (11 men, 8 women;
mean age=23.2 [+ or -] 2.3 years, height=168.2 [+ or -] 7.3 cm,
weight=61.3+9.7 kg). Volunteers were included if they had no history of
low back pain or lower extremity injuries, such as sprains or fractures,
and were able to maintain a 5-second single-legged hold on the floor
(stable condition) and on a round foam roll (unstable condition). They
were excluded if they had a prior low back or lower extremity surgery,
leg-length discrepancy, marked kyphosis or scoliosis, or neurologic
disease. The dominant leg was determined by asking the participant to
kick a soccer bali; the kicking leg was determined to be the dominant
leg. (17-19) All participants were right-leg dominant. The
university's institutional review board approved the study, and all
volunteers provided written informed consent before the study began.

Electromyography

Surface electromyography (EMG) was used to measure muscle activity.
The EMG data were collected bilaterally from the RA, EO, and TrA/IO
muscles (Figure 1). Electrode placement for each muscle is described in
Table 1. (20, 21) Electrode placement for the TrA/IO was based on
previous reports. (22, 23) McGill et al (22) stated that the EMG signal
obtained from an electrode inferior to the anterior-superior iliac spine
represents the combined activity of the TrA and IO. In an ultrasonic
imaging study of 10 cadavers, Marshall and Murphy (23) confirmed that
the TrA and IO muscles were fused 2 cm medial and inferior to the
anterior-superior iliac spine and reported no overlap of the EO muscle.

The skin was shaved, sanded, and swabbed with alcohol-soaked cotton
before electrode placement to minimize skin resistance. (2) A small
amount of electrode gel was applied to silver chloride electrodes
(DE-3.1 double differential electrode; Delsys, Inc, Boston, MA), which
were then applied to the skin. (2) The reference electrode was applied
to the lateral malleolus of the dominant leg. The EMG data were
collected using a data acquisition system (model MP100WSW; Biopac
Systems, Inc, Goleta, CA). (2)

The analog signals were converted to digital signals, and the
converted signals were processed for analysis using Acknowledge software
(version 3.8.1; Biopac Systems, Inc). The raw EMG signal was recorded at
a sampling rate of 1000 Hz. A bandpass filter of 20-450 Hz was used to
eliminate movement artifact, and a 60-Hz notch filter was used to
minimize electrical noise. (2, 20) The EMG signal was processed to the
root mean square (RMS) using a moving window of 50 milliseconds and
analyzed as an ASCII file.

[FIGURE 1 OMITTED]

For normalization, the RMS of a 5-second maximal voluntary
isometric contraction (MVIC) was measured 3 times for each muscle in the
standardized position (Table 2). (24) The average RMS of 3 measurements
was used to determine the MVIC of each muscle.

For the single-legged hold measurement, the data were collected
during a 5-second period. Data from the initial 1 second and final 1
second were excluded; thus, 3 seconds of data were analyzed. A 1-minute
rest period was provided between measurements to prevent muscle fatigue.
The normalized muscle activity was expressed as a percentage of the MVIC
(%MVIC = [average RMS on the floor or on a round foam roll/ average RMS
of 3 MVICs] x 100).

Procedures

Each participant was instructed to lie supine on either the floor
or a round foam roll (15.2 x 91.4 cm; Sammons Preston Rolyan,
Bolingbrook, IL). Using a universal manual goniometer (Sammons Preston
Rolyan), the principal investigator (S.J.K.) measured hip and knee joint
angles to place the participant in the hook-lying position. The hip and
knee joints bilaterally were flexed to 45[degrees] and 70[degrees],
respectively, so that the lower back was flat on the floor or the round
foam roll. The hip and knee joint angles of the dominant leg (supporting
leg) were maintained at 45[degrees] and 70[degrees], respectively,
during the single-legged hold of the nondominant leg, in both the floor
and the round foam roll conditions (Figure 2). A target bar was placed
so that the participant's ankle would touch it with full extension
of the knee joint. Elastic guides were aligned with the lower extremity
to limit abduction of the hip and adduction of both legs (Figure 3).
When the examiner observed a deviation from the vertical elastic guides
resulting from excessive pressure, the data were discarded. A small box
(30.5 x 40.6 x 15.2 cm) the same height as the round foam roll was
placed under both feet during knee extensions performed on the round
foam roll; this was to ensure that the hip and knee joint angles were
the same as those for knee extensions performed on the floor (Figure
2B).

The familiarization period consisted of approximately 1 hour
(25-minute session with 10-minute break between sessions). During the
familiarization period, the participant was instructed to raise the
nondominant lower limb until he or she touched the target bar on the
dorsum of the foot without pressing the vertical elastic guide. The
participant was instructed to use the fingertips of both hands to touch
the floor and maintain balance without falling off the foam roll. The
amount of support from the fingertips decreased gradually as the
participant became familiar with the foam roll. The familiarization
period was completed when the participant was able to maintain 3
consecutive 5-second single-leg holds with 1-minute rest periods without
fatigue on a round foam roll. All participants felt comfortable, and
none reported fatigue after the familiarization period. A 15-minute rest
period after familiarization was allowed before data collection.

[FIGURE 2 OMITTED]

Performance of the single-leg hold was randomized by selecting from
the numbers 1 and 2 (number 1, floor; number 2, round foam roll). The
participant extended the nondominant knee joint until the ankle joint
touched the target bar and then sustained an isometric contraction for 5
seconds. During the unstable condition, the participant was asked to lie
on the round foam roll. The head and vertebral column were aligned to
the longitudinal axis of the round foam roll. Then the participant was
asked to extend the nondominant knee without moving the hip. He or she
was instructed to hold the nondominant leg steady at the target position
without falling off the round foam roll. Data were collected when the
participant maintained the test position while holding the leg within
the vertical elastic guides and without loss of balance. Three trials
were performed with a 1-minute rest period between trials. A 3-minute
rest period was provided between conditions when changing from one
supporting surface to the other to minimize muscle fatigue. (24)

Statistical Analyses

A paired t test with Bonferroni adjustment was used, with the level
of significance set at P=.008 (.05/6) to compare muscle activity
generated during exercise performed on the floor and the round foam
roll. The effect size was calculated using the pooled SD. Data were
processed with SPSS (version 12.0; SPSS Inc, Chicago, IL). The
percentage increment of muscle activity across the supporting surface
was determined ([muscle activity difference between the floor and the
round foam roll/muscle activity on the floor] x 100).

[FIGURE 3 OMITTED]

RESULTS

We observed greater abdominal muscle activity during a
single-legged hold on the round foam roll than on the floor (Table 3).
The percentage increment of muscle activity was 88.08% in the
contralateral RA ([P.sub.adj] =.003), 107.81% in the ipsilateral RA

([P.sub.adj] DISCUSSION

We compared the amplitude of EMG activity of bilateral abdominal
muscles during a single-legged hold exercise performed on the floor and
on a round foam roll. The single-legged hold exercise on a round foam
roll led to greater EMG activity levels in the bilateral RA, EO, and
TrA/IO muscles than did the floor condition.

Two possible explanations exist for greater muscle activity on the
round foam roll. First, because of the instability of the round foam
roll supporting surface, muscles crossing the abdominal area need to
contract together to maintain stability during the single-leg hold.
Vera-Garcia et al (13) reported that RA and EO muscle activity on a gym
ball (unstable surface) was greater than that on a stable bench during
the curl-up exercise. Marshall and Murphy (14) reported that the RA and
TrA/IO muscle activity on a gym ball was greater than on a stable
surface during a press-up exercise. Our findings are in accordance with
those of previous researchers demonstrating greater muscle activity on
unstable surfaces than on stable surfaces. (13-15) Second, when
participants lie on a round foam roll as compared with the floor, the
contact area is smaller. Santos and Aruin (25) demonstrated that
maintaining the center of gravity within a reduced base of support was
more challenging and necessitated EO muscle contraction. They also noted
that muscle coactivation can lead to increased joint stiffness,
assisting counterbalancing body perturbations. (25) Therefore, lying on
a smaller base of support on a round foam roll could have induced more
abdominal muscle activity than lying on the floor.

During a single-legged hold on a round foam roll, bilateral TrA/IO
muscle activity was greater than that of the RA and EO. A TrA/IO
contraction increases intra-abdominal pressure; together these factors
play key roles in maintaining lumbar segmental stabilization because the
IO muscle blends with the lateral raphe of the thoracolumbar fascia. (1,
9, 22, 23, 26, 27) During the single-legged hold, trunk stability is
further challenged by the unstable surface, and bilateral TrA/IO muscle
activity was thought to be induced to maintain stability. Thus, the
single-legged hold performed on a round foam roll increases abdominal
activity, including bilateral contraction of the TrA/IO, a lumbar
stabilizer. (2)

Although all abdominal muscle activity increased during the
single-legged hold on the round foam roll, it is interesting that the
muscle activity of the contralateral TrA/IO was greater than that of the
ipsilateral TrA/IO, whereas for the RA and EO, the ipsilateral muscle
showed a greater level of activity. These results are also consistent
with previous findings that demonstrated greater activity of the
contralateral TrA/IO and ipsilateral EO during a single-leg lift in
4-point kneeling, (26) which created a rotation moment toward the side
of the single-legged lift. Thus, a stable pelvis and spine posture arose
from cooperation of the contralateral IO and the ipsilateral EO. It is
possible that increased contralateral IO activity was caused by
counteracting movement induced by the single-legged hold on an unstable
surface. Behm et al (15) reported that the unilateral chest press using
a dumbbell was more effective in activating all trunk stabilizers than
was a bilateral arm exercise. They suggested that the unbalanced
movement of a unilateral arm outside the base of support would result in
a destabilizing torque that was counteracted by a contralateral trunk
muscle contraction. (15) Liebenson et al (1) measured the lumbar axial
rotation during active straight-leg raising in asymptomatic volunteers
using an electromagnetic tracking device and found 5.4[degrees] of
lumbar axial rotation with no abdominal bracing. With the single-legged
hold of the ipsilateral side performed on a round foam roll in our
study, the roll would rotate to the ipsilateral side secondary to the
tendency toward lumbar rotation that has been observed previously (1) in
the straight-leg raise and increased rotation toward the side of the
single-legged hold. Therefore, we believe that to counterbalance the
ipsilateral rotation and maintain trunk stability, the contralateral
TrA/IO, EO, and RA cocontract to produce a contralateral rotation
movement to maintain trunk balance and avoid falling. (15) In
particular, the contralateral TrA/IO seemed to be activated more as a
lumbopelvic stabilizer. Because instability cannot be overcome by the
contralateral TrA/IO, the ipsilateral TrA/IO begins to contract along
with the contralateral TrA/IO to counterbalance the rotation force
toward the ipsilateral side and to improve lumbar stability by
increasing intra-abdominal pressure via bilateral TrA/IO contractions.
(1, 27-30) These findings are also thought to be elicited by a diagonal
trunk rotation moment; the ipsilateral EO and contralateral TrA/IO
contract together to counterbalance the ipsilateral rotation moment on a
round foam roll. However, we think further research is needed to clarify
the exact mechanism for greater contralateral than ipsilateral TrA/IO
activity.

In our study, ipsilateral RA and EO muscle activity was greater
than that on the contralateral side. This greater ipsilateral RA and EO
muscle activity can be attributed to their synergistic roles in
minimizing compensatory pelvic rotation. (31) When the single-legged
hold is maintained on a round foam roll, the hip flexors contract
isometrically. This contraction of the hip flexors may tend to tilt the
pelvis anteriorly. Therefore, the ipsilateral RA and EO are likely to
contract synergistically for pelvic stability.

Some authors have demonstrated a leg dominancy effect on the
strength of the hip muscle, (18) joint torque, and ground reaction force
during a squat. (32) Conversely, squat strength (17) and postural
control (eg, sway area and sway path length in single-legged standing)
(19) did not differ between the dominant and nondominant legs. (19) In
our study, all participants were right-leg dominant, so there may be a
leg-dominance effect on trunk muscle activity. Future study is needed to
investigate the effect of leg dominancy on abdominal muscle activity
during the single-legged hold on the round foam rolL We recruited
healthy volunteers without a history of low back pain or lower extremity
injury; thus, our findings cannot be generalized to patient populations.
We did not directly measure the lumbopelvic rotation angle, and further
study is needed to examine the lumbopelvic rotation during the
single-legged hold task. The test position in this study consisted of
the participants lying on their backs, so it was almost impossible to
position electrodes for measuring the EMG activity of the back muscles.
McGill et al (22) stated that the surface electrode position over the IO
and TrA demonstrated the fine-wire activity of the TrA within
approximately 15% of the contraction amplitude. To our knowledge, no
authors have reported a standard method of measuring the MVIC of the TrA
with surface EMG. This limitation might have affected the results of the
TrA/IO muscle activity in our study. Our investigation should be
replicated in patient populations to generalize the findings, and
longitudinal studies should be performed to determine the long-term
effect of the single-legged-hold exercise on a round foam roll on muscle
activity.

CONCLUSIONS

The activity of all the abdominal muscles measured increased during
a unilateral single-legged hold exercise performed on a round foam roll.
This finding suggests that performing the single-legged hold exercise on
an unstable round foam roll is more effective in recruiting abdominal
muscle activity than is exercise on a stable surface.

Key Points

* The unilateral single-legged hold exercise performed on a round
foam roll resulted in greater abdominal muscle activation than did the
same exercise performed on a stable surface.

* During this exercise on a round foam roll, activation of the
transversus abdominis/internal oblique muscles was greater on the
contralateral side, but activation of the rectus abdominis and external
oblique muscles was greater on the ipsilateral side.

ACKNOWLEDGMENTS

This study was supported in part by the Korea Research Foundation
Grant, funded by the Korean government (Ministry of Education, Science
and Technology, Basic Research Promotion Fund).